1. Field of Invention
This invention relates generally to the molding of plastic objects, and more particularly to a mold assembly usable by children to produce in a microwave oven plastic play pieces, and to a safety container for housing the mold assembly to prevent access thereto until the assembly has cooled to a temperature at which it is safe to handle.
2. Status of Prior Art
Plastics are organic substances made synthetically by polymerization and capable of being formed or molded into various products. The term "resin" is generally applied to the initially made polymeric material, whereas the term "plastic" is normally reserved for products made from resin which incorporates other materials, such as fillers, coloring agents, anti-oxidants and plasticizers.
The present invention, which is directed to the molding of plastic objects, uses for this purpose any resin incorporating a substance rendering the resulting composition flowable so that it can be poured into the cavity of a mold and then heated to a temperature at which the plastic cures and hardens to form a molded plastic piece. The Curie temperature is that temperature at which a resin undergoes curing, which is the process by which the hot liquid resin sets to a solid at the same temperature. The amount of time it takes for curing to occur depends on the nature of the composition.
By way of example, the invention will be described in connection with a plastisol having any predetermined curing temperature. It is to be understood, however, that the invention encompasses the use of a plastic molding composition having the necessary characteristics and is not limited to plastisols.
A plastisol is a dispersion of finely divided resin in a plasticizer such as an organic, non-volatile liquid. Plastisols are commonly used in molding thermoplastic resins, chiefly polyvinyl chloride (PVC). Thus, to produce a molded PVC piece, the plastisol is poured into the cavity of a mold, and the mold is placed in an oven in which the plastisol is heated to an elevated temperature whose level depends on the curing point of the plastisol; that is, the temperature at which the plastisol cures and hardens to form a solid plastic piece whose form conforms to the shape of the mold.
The concern of the present invention is with a mold assembly usable by children to produce in a standard microwave oven normally intended to heat and cook food, molded plastic play pieces. It is essential, therefore, that the nature of the assembly be such that it can be safely handled by a child even though the elevated temperature necessary to carry out a molding operation can be injurious, unless the child is shielded from the hot mold.
Taken into account in the invention is that a child is not in a position to set a microwave oven so that it is turned off automatically when the molded plastic pieces within the oven have cured and hardened. The child has no knowledge of the curing temperature of the plastic composition being molded, nor can he see within the oven to determine whether the pieces are finished. And after molding has been completed within the microwave oven and the oven is turned off, the mold containing the plastic pieces is still very hot, and a child would be ill advised to then try to take the mold out of the oven.
The use of microwave ovens to heat or cook food is now commonplace, and microwave ovens are installed in many households. In the typical microwave oven, a magnetron functions to generate microwave energy at a frequency of about 1000 mHz. This energy is conveyed by a wave guide to the interior of the oven to irradiate the food placed therein. Because food absorbs microwave energy, this gives rise to internal molecular friction which heats the food at a rate that depends on its "lossy" characteristics. Some food products are heated more rapidly than others in a microwave oven; but in general the cooking or heating of food by microwave energy is much faster than by conventional heating techniques, including infrared radiation. In heating or cooking food in a microwave oven, the food is placed in a receptacle of synthetic plastic, glass or other material which is non-reactive to microwave energy; hence, it is only the food that is heated.
U.S. Pat. No. 3,941,967 discloses a microwave cooking apparatus capable of scorching the surface of the food being cooked without excessively heating the interior of the food. This apparatus, which is put into a microwave oven, is in the form of a casing, within which is disposed a plate in which is placed the food to be cooked. Below the plate is a thermal heating element which generates heat by absorption of microwave radiation, use for this purpose being made of a ferrite ceramic. Thus, the interior of the food is heated by the microwave energy absorbed thereby, while at the same time the exterior of the food is thermally heated and scorched by the plate which is heated by the ferrite heating element.
A similar arrangement is shown in U.S. Pat. No. 4,496,815, in which a microwave browning utensil includes a metal platter on which the food to be heated in the microwave oven is placed. On the underside of the platter is a thermal heating element formed by powdered ferrite dispersed in a matrix of organic material. In this way, the interior of the food on the platter which absorbs microwave energy is heated and cooked, while the exterior of the food is thermally heated and browned. Thermal heating takes place mainly by conduction; hence, the exterior of a body being heated is first subjected to the heat before it penetrates the interior of the food body, whereas with microwave heating, the radiation penetrates the interior of the body.
U.S. Pat. No. 4,266,108 discloses the use of a ferrite adjacent a microwave reflecting member in which the ferrite material acts as a heating element that will rise in temperature to a predetermined level which depends on the Curie point of the ferrite.
The Curie point of a ferrite is the temperature marking the transition between ferromagnetism and paramagnetism. When in its ferromagnetic state, the ferrite then absorbs microwave energy and is heated thereby. This action ceases when the ferrite enters its paramagnetic state. Hence, when a ferrite heating element is placed in a microwave oven and is subjected to microwave energy, the heating element will become increasingly hot until an elevated temperature is reached that depends on the Curie point of the ferrite, after which no more heat is generated even though the microwave oven is still operating. Thus, the ferrite heating element will effectively be turned "off," even though the microwave oven is still "on."